President Donald Trump’s plan to build a wall on the border with Mexico is facing a major problem: A wall of resistance from his own party.

A growing number of congressional Republicans are objecting to the cost and viability of a proposal that was a rallying cry for the billionaire businessman during his insurgent campaign. Interviews with more than a dozen GOP lawmakers across the ideological spectrum suggest Trump could have a difficult time getting funding for his plan approved by Congress.

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Many bluntly told CNN they’d likely vote against any Trump plan that is not fully offset with spending cuts, while others questioned whether Trump’s vision would adequately resolve the problems at the border.

“If you’re going to spend that kind of money, you’re going to have to show me where you’re going to get that money,” said Alaska Sen. Lisa Murkowski, a key swing vote who has already broken with Trump over his nominee for secretary of education.